Investment casting techniques innovated in the late 1960s have revolutionized the design, construction and performance of golf club heads up to the present time. Initially only novelty putters and irons were investment cast, and it was only until the early years of the 1980s that investment cast metal woods achieved any degree of commercial success. The initial iron club heads that were investment cast in the very late 1960s and early 1970s innovated the cavity backed club heads made possible by investment casting which enabled the molder and tool designer to form rather severe surface changes in the tooling that were not possible in prior manufacturing techniques for irons which were predominantly at that time forgings. The forging technology was expensive because of the repetition of forging impacts and the necessity for progressive tooling that rendered the forging process considerably more expensive than the investment casting process and that distinction is true today although there have been recent techniques in forging technology to increase the severity of surface contours albeit them at considerable expense.
The investment casting process, sometimes known as the lost wax process, permits the casting of complex shapes found beneficial in golf club technology, because the ceramic material of the mold is formed by dipping a wax master impression repeatedly into a ceramic slurry with drying periods in-between and with a silica coating that permits undercutting and abrupt surface changes almost without limitation since the wax is melted from the interior of the ceramic mold after complete hardening.
This process was adopted in the 1980s to manufacture "wooden" club heads and was found particularly successful because the construction of these heads requires interior undercuts and thin walls because of their stainless steel construction. In order to conform to commonly acceptable club head weights in both woods and irons, it was difficult to enlarge the ball striking surface. This ball striking surface, even utilizing a high strength stainless steel such as 17-4, without reinforcement, must have a thickness of at least 0.125 inches in metal woods to even heavier in irons to maintain its structural integrity for the high club head speed player of today who not uncommonly has speeds in the range of 100 to 150 feet per second at ball impact.
Faced with this dilemma of manufacturing a club head of adequate strength while limiting the weight of the club head, designers have found it difficult to increase the perimeter weighting effect of the club head.
Since the innovation of investment casting in iron technology in the late 1960s, perimeter weighting of iron clubs has been achieved by a redistribution of the weight of the head itself away from the hitting area to the perimeter around the hitting area, usually by providing a perimeter wall extending rearwardly from the face that results in a rear cavity behind the ball striking area. Such a club head configuration has been found over the last two plus decades to enable the average golfer, as well as the professional, to realize a more forgiving hitting area and by that we mean that somewhat off-center hits from the geometric center of the face of the club results in shots substantially the same as those hits on the center of the club. Today it is not uncommon to find a majority of professional golfers playing in any tournament with investment cast perimeter weighted irons confirming the validity of this perimeter weighting technology.
In the Raymont, U.S. Pat. No. 3,847,399 issued Nov. 12, 1974, assigned to the assignee of the present invention, a system is disclosed for increasing the perimeter weighting effect of a golf club by a pattern of reinforcing elements in the ball striking area that permits the ball striking area to be lighter than normal, enabling the designer to utilize that weight saved on the forward face by adding it to the perimeter wall and thereby enhancing perimeter weighting.
This technique devised by Mr. Raymont was adopted in the late 1980s by many tool designers of investment cast metal woods to increase the strength of the forward face of the metal woods to maintain the requirement for total overall head weight and to redistribute the weight to the relatively thin investment cast perimeter walls permitting these walls to not only have greater structural integrity and provide easier molding and less rejects, but also to enhance the perimeter weighting of these metal woods. Most major companies in the golf industry manufacturing metal woods in the late 1980s were licensed under the Raymont patent.
In 1991, the Allen, U.S. Pat. No. 5,060,951 issued entitled "Metal Headed Golf Club With Enlarged Face", also assigned to the assignee of the present invention, and it discloses an investment cast metal wood with an enlarged club face depth(height) on the order of at least 1.625 inches. Such a face depth was not formerly believed possible because of the requirement for face structural integrity under the high impact loads at 100 to 150 feet per second, and the weight requirement of the club head of 195 to 210 grams. In this Allen patent, a labyrinth of reinforcing elements similar to Mr. Raymont's was utilized not to re-distribute face weight but instead to enlarge face area while maintaining overall club head weight. An ancillary and important advantage of this development, utilized by many present day designers of "jumbo" metal wood heads, is the fact that an enlarged club face produces a sweet spot enlargement far greater than the enlargement of the club face itself.
Another problem addressed by the present invention is the achievement of increasing the benefits of perimeter weighting by simply adding weight to the perimeter of the club head itself. This technique of course has found considerable success in low impact club heads such as putters, where overall club head weight is in no way critical, and in fact in many low impact clubs that have found considerable commercial success, the club heads weigh many times that of metal wood heads, sometimes three or four times as heavy.
To this date, however, increased perimeter weighting has not been found easy because of the weight and impact strength requirements in high impact clubs. An understanding of perimeter weighting must necessarily include a discussion of the parameter radius of gyration. The radius of gyration in a golf club head is defined as the radius from the geometric or ball striking axis of the club along the club face to points of club head mass under consideration. Thus, in effect the radius of gyration is the moment arm or torquing arm for a given mass under consideration about the ball striking point. The total moments acting on the ball during impact is defined as the sum of the individual masses multiplied by their moment arms or "radii of gyration". And this sum of the moments can be increased then by either increasing the length of the individual moment arms or by increasing the mass or force acting at that moment arm or combinations of the two.
Since it is not practical, except for the techniques discussed in the above Raymont and Allen patents, to add weight to the perimeter wall because of weight limitations, one alternative is to increase the moment arm or radius of gyration. This explains the popularity of today's "jumbo" woods although many of such woods do not have enlarged faces because of the requirement for structural integrity in the front face.
In the Allen, U.S. patent application, Ser. No. 882,561, now U.S. Pat. No. 5,301,941, some of these problems are solved with a composite face wall that includes an impact supporting wall that is investment cast with the remainder of the head(without the sole plate which is a separate piece as cast). This impact supporting wall is rigidified by a pattern of integrally cast reinforcing bars that extend forwardly from the forward wall rather than rearwardly as described in the above discussed Raymont and Allen patents. This reinforcing pattern has a depth of approximately 0.150 inches which is significantly greater than reinforcing patterns possible on the rear of the ball striking faces of prior constructions. This increased depth provides far greater supporting wall reinforcement. It is also easily cast because the core piece that forms these deep depth reinforcing elements are removed by a direct forward withdrawal unencumbered by the perimeter wall that inhibits rearward core withdrawal inside the club head. In the exemplary embodiment of this pattern of reinforcing bars, the reinforcing bars are formed into hexagonal unit cells having a major diameter of 0.500 inches, although other geometric patterns are within the scope of the present invention.
This reinforced supporting wall is covered by a very hard plastic ball striking insert that is cast in situ(in place) over the supporting wall. That is, after the head is investment cast, the forward wall is cleaned and vulcanized with a bonding agent and placed in a mold that carries the configuration of the outer surface of the insert and an elastomeric material is either poured or injected under pressure into the mold to form the insert. One material that has been found successful is a Shore D 75 hardness polyurethane, which results in a very hard high frequency ball striking surface. This plastic insert, not only provides a very hard ball striking surface, but more importantly because it is intimately bonded to the forward wall and the reinforcing bars, it provides an effective "I" beam support with the bars for the forward wall as opposed to a "T" beam support found in today's rearwardly reinforced ball striking wall. It can be easily demonstrated by engineering calculation that I beam supports for transverse loads are substantially stronger than T beam supports.
The increase in the radius of gyration is accomplished by extending the heel and toe portions of the club head further from the geometric center of the face wall, beyond present day parameters for high impact club heads. These extensions provide greater effective heel and toe weighting. The heel of the club head is formed by extending tending the club face significantly beyond the hosel, that is, on the side of the hosel opposite the ball striking area, and extending the top wall and rear wall to accommodate this extended face.
In my U.S. patent application entitled "METAL WOOD GOLF CLUB WITH TRUE HEEL AND TOE WEIGHTING", U.S. application Ser. No. 022,902, filed: Feb. 26, 1993, an improved metal wood golf club is provided having an enlarged or "jumbo" metal club head with a crowned top wall extending rearwardly from a ball striking face wall, a toe wall, and a heel wall also projecting rearwardly from the face wall--but the club head has no conventional sole plate.
In that design, the toe wall and the heel wall are enclosed by the top wall and a pair of spaced generally vertical weighting walls integral with and extending rearwardly from the face wall. The two areas enclosed by the top wall, heel and toe walls, and weight walls are hollow to achieve the desired head weight and the area between the walls is opened, and the weight of the sole plate that normally encloses that area is redistributed to the weight wall to achieve true heel and toe weighting.
Prior attempts to manufacture very large stainless steel metal club heads with larger than normal faces has proved exceedingly difficult because of weight requirements for club heads to achieve the most desirable club swing weights. Thus, to the present date stainless steel "jumbo" club heads have for the most part been manufactured with standard sized face walls. This has led to several manufacturers switching from stainless steel to aluminum alloys, which are of course lighter, to enlarge the head as well as the face.
Face enlargement was achieved in the design disclosure in my U.S. patent application, Ser. No. 022,904, by a combination of a honeycomb reinforcing network formed integrally on the rear surface of the forward wall between the weighting walls and a redistribution of the weight of the conventional sole plate, which is eliminated in that design, and the weight saved on the thinner than normal face wall to the weighting walls themselves. The two enclosed areas defined by the top wall, heel and toe walls, and weighting walls were hollow.